The yeast, Saccharomyces cerevisiae, will be used to determine how the chromosome assembles and is topologically altered during the cell cycle. The following distinct but related steps in formation of the dynamic chromosome will be studied. a) The cis- and trans-acting factors involved in controlling histone synthesis and stoichiometry in S phase will be identified. b) Strains in which individual core histones H2B and H4 are repressible by GAL promoters will be used to determine the steps in histone assembly, how nucleosomes segregate during DNA replication and whether gross changes in chromosome structure affect transcription. c) Deletions in the charged histone ends will also be analyzed by conditional synthesis to determine their effects on assembly of the nucleosome and changes in chromosome structure. d) Since N-terminal acetylation of histones has been implicated in assembly and structural changes of the chromosome, we will study this process by site-directed mutagenesis of acetylated lysines. Also, we will use a cloned gene (HAT-c), whose mutation results in decreased histone acetylase activity, to probe the function of acetylation during the yeast cell cycle.